Add constructor with ToastType only

[NathanJPhillips]

Add a constructor for ToastMessage that takes ToastType only.

With the new RenderFragment toast content a constructor that takes ToastType but not a string message now makes sense.

At the same time make some changes to other constructors:
o Invoke simpler constructors from more complex ones to avoid duplicating code
o Use default argument for autoHide instead of writing two separate constructors
o Add autoHide parameter to constructor that takes IconName to mirror the constructor that takes customIconName